8th Grade Science – Professor Shai

Fractional Distillation Lab

Section 5.1 – Page 90

Under Construction

 

Introduction

Fractional distillation is a way to separate liquids in a mixture by using the characteristic property of their boiling points.  Essentially, you boil off the liquid and collect the gas by condensing it in cooled test tubes.  If the different liquids in the mixture have distinct boiling points, then each will boil out at different temperatures.  The trick is knowing when to switch test tubes.  This can be determined by using a thermometer, recording temperatures, graphing them and figuring out the different boiling points - i.e.,  when the graph goes flat. 

Goals

The goals of this experiment are to distill a mixture of fluids, separate the components, and perform tests to identify each one.

Method

This is a long experiment that will take you about a week to complete.  It is an important experiment because you will need to use these techniques in the sludge project coming up.  It is divided into parts A-F, and you should attempt to complete at least one part each day.

Part A:  Test the mixture in four ways:


Record all your results.  Write them on the board.  Answer all blue questions in the book.  

Technique:  For the next two parts you will use the apparatus shown on page 91 of your book.  Here are some notes on technique.  Note that the bulb of the thermometer is above the boiling liquid in Part B, and below it in Part C.  Also, note that the end of the rubber tube in the collecting test tubes is below the level of the cooling liquid and above the level of the collected liquid.  This prevents distilled liquid from going back up the rubber tube into the boiling test tube. Finally, to get good results you need to have even steady heating.  To make sure you don't "bump" the temperature, use fresh boiling chips every time you start a new boil, and adjust the screen under the tube to allow more or less heat as needed.

Part B:  Do a dry-run distillation on 5cc's of the mixture to determine the number of component liquids and boiling points. 
Record the temperature of the vapors every minute.  The thermometer should be above the boiling fluid in the vapors.  Graph the temperature versus time and determine how many distinct boiling points there are, and what temperatures they are at.  Answer all blue questions in the book.

Part C:  Distill 25 cc's of the mixture into collecting tubes. 

Verify the temperatures by recording the temperature of the liquid (not the vapors) every minute, and graph the results.  The thermometer should be in the boiling liquid this time.  You should collect a new tube a few seconds after the boiling point temperature of each substance is reached, i.e., just to the right of the start of each flat section on your graph.  Also, start a new tube after each time the temperature starts to rise again, i.e.  just to the right of the end of each flat section on your graph.  This allows you to capture each liquid as well as a mixture of each pair of liquids.  For example, if your mixture is made up of just two liquids, I and II, you collect 3 tubes: I, I and II, and II.  If your mixture had 3 liquids originally I, II, III, then you collect 5 tubes:  I, I and II, II, II and III, and III,  Compare your temperature graph with the graph from Part B.  Label each collecting tube with your group name and the appropriate I or II etc.  When you are finished, test each liquid (also called fraction) for odor and flammability.   Record all results.  Save your samples.


Parts D-E:  For each test tube of fluid you collected:

  • Test whether or not sugar is soluble in your liquid.  Write results on the board.
  • Measure the density. Write results on the board.
  • Boil the whole sample and recollect it, creating a vapor-temperature graph (thermometer above the liquid) as you proceed.

Part F: Make a chart summarizing all your results.  Write results on the board.  For each test tube, your chart should include:
  • Odor (Strong, Weak, None)
  • Flammability (Yes, No)
  • Density (Numerical value in g/cc)
  • Solubility of Sugar (Yes, No, Slightly)
  • Boiling Point (Numerical value in degrees Celsius, or not constant).

Answer all blue questions on page 92 in the book.


Conclusions

The original mixture was made up of two liquids, isopropanol (rubbing alcohol) and water.  Test tube I was mostly isopropanol, test tube I-II was a mixture of isopropanol and water, and test tube II was mostly water.  A chart showing sample good results is shown below:

Liquid
Odor
Flammability
Density g/cc
Solubility of Sugar
Boiling Point in C
Original Mixture
Weak
Yes
.91
Soluble
not constant
Tube I
Strong
Yes
.84
Insoluble
80
Tube I-II
Weak
Yes
.93
Slightly Soluble
not constant
Tube II
None
No
1.03
Soluble
100



Book Problems

Do problems 1-3, on page 92.  Ask me for help if you are not sure how to do them.